Semiconductor light sources such as, for example, light-emitting diodes have increased in interest during recent years for lighting applications. The reason for this consists inter alia in that critical technical innovations and significant advancements both in terms of brightness and in terms of light efficiency (luminous efficacy per watt) of these light sources could be achieved. Not least owing to the comparatively long life, light-emitting diodes could develop as an attractive alternative to conventional light sources such as incandescent lamps or gas discharge lamps.
Semiconductor light sources are sufficiently well known from the prior art and will be abbreviated below as LED (light-emitting diode). This term is intended to include both light-emitting diodes consisting of inorganic materials and light-emitting diodes consisting of organic materials in the text which follows. It is known that the light emission from LEDs correlates with the current flow through the LEDs.
For brightness regulation, LEDs are therefore in principle operated in a mode in which the current flow through the LED is regulated.
In practice, in order to actuate an arrangement of one or more LEDs, preferably switching controllers, for example step-down converters (buck converters) are used. Such a switching controller is known, for example, from DE 10 2006 034 371 A1. In this case, a control unit actuates a high-frequency-clocked switch (for example a power transistor). In the switched-on state of the switch, current flows via the LED arrangement and a coil, which is charged. The buffer-stored energy in the coil discharges in the switched-off state of the switch via the LEDs (freewheeling phase). The current through the LED arrangement demonstrates a zigzag-shaped time profile: when the switch is switched on, the LED current demonstrates a rising edge, and when the switch is switched off, a falling edge results. The mean value of the LED current over time represents the rms current through the LED arrangement and is a measure of the brightness of the LEDs. By corresponding clocking of the power switch, the mean, rms current can be regulated.
The function of the operating device now consists in setting a desired mean current flow through the LEDs and keeping the fluctuation range of the current over time as low as possible, determined by the high-frequency connection and disconnection of the switch (typically in the region above 10 kHz).
A large fluctuation range of the current (ripple) has a disadvantageous effect particularly in the case of LEDs since, with the change in the current amplitude, the spectrum of the emitted light can change.
In order to keep the emitted light spectrum as constant as possible during operation, it is known not to vary the current amplitude in the case of LEDs for brightness regulations, but to use a so-called PWM (pulse width modulation) method. In this case, low-frequency (typically with a frequency in the range of 100-1000 Hz) pulse packets with a constant (when averaged over time) current amplitude are supplied to the LEDs by the operating device. The abovementioned high-frequency ripple is superimposed on the current within a pulse packet. The brightness of the LEDs can now be controlled by the frequency of the pulse packets; the LEDs can be dimmed, for example, by the time interval between the pulse packets being increased.
A practical demand made of the operating device consists in that it can be used as flexibly and in as versatile a manner as possible, for example irrespective of how many LEDs are actually connected as load and are intended to be operated. The load can also change during operation when, for example, an LED fails. In conventional technologies, the LEDs are operated in a so-called “continuous conduction mode”. This method will be explained in more detail with reference to FIG. 1a and FIG. 1b (prior art).
In the example shown in FIG. 1a, a step-down converter (buck converter) is illustrated as basic circuit for the operation of at least one LED (or a plurality of LEDs connected in series), which has a first switch S1. The operating circuit is supplied a DC voltage or a rectified AC voltage U0.
In the switched-on state of the first switch S1 (during the time period t_on), energy is built up in the coil L1 and is discharged in the switched-off state of the first switch S1 (time period t_off) via at least one LED. The resultant current profile over time is depicted in FIG. 1b (prior art). In this case, two pulse packets of the PWM are illustrated. The current profile within a pulse packet is additionally illustrated in enlarged form. For reasons of color constancy, the amplitude of the ripple should be as small as possible within a pulse packet. This can take place by suitable selection of the switch-on time t0 and the switch-off time t1. Thus, these times can be selected, for example, in such a way that the first switch S1 is switched on when the current undershoots a specific minimum reference value, and the switch is switched off when the current exceeds a maximum reference value. This method has several disadvantages, however: firstly, in order to achieve as little ripple as possible, a rapid sequence of switch-on and switch-off operations is required. The gradient (positive or negative edge) of the current is not controllable by the operating device and should be considered as being given since it is determined, inter alia, by the inductance of the coil L1 and by the power consumption of the LEDs.
Owing to tolerances in the components of the operating circuit and also owing to the limited resolution of the clock units, flicker phenomena or other disturbances can arise.